It is known to provide an electronic stability control (ESC) system (sometimes also referred to as a dynamic stability control (DSC) system) for reducing a risk of loss of traction of a motor vehicle. ESC systems have been found to be particularly effective in recovery of a vehicle from a loss of traction scenario when cornering.
ESC systems typically monitor steering wheel angle or steerable road wheel angle and vehicle direction and determine a difference (or error) between the drivers intended direction of travel (determined through the measured steering wheel angle) and the vehicle's actual direction (determined through measurements of lateral acceleration, yaw and the speed of rotation of each road wheel).
If the ESC system determines that the error exceeds a prescribed value indicating that the vehicle is skidding about the vehicle's vertical axis, the system estimates the direction of the skid and then applies braking to individual wheels in order to create torque about the vertical axis opposing the skid. In some arrangements the system also slows the vehicle, for example by reducing engine power.
ESC systems employ the anti-lock braking system (ABS) to brake individual wheels as required. Furthermore, a traction control system (TCS) may be incorporated into the ESC system. TCS systems detect drive wheel slip under acceleration and apply negative torque (braking) to the slipping wheel and/or reduce engine power until traction is regained.
ESC systems typically receive signals corresponding to steering wheel position, lateral acceleration, yaw and a speed of rotation of each wheel.
In addition, in some known arrangements the ESC system also receives data corresponding to a value of roll rate of the vehicle and to a ratio of the value of the longitudinal acceleration of the vehicle to the value of the coefficient of friction between wheels of the vehicle and the driving surface The value of the coefficient of friction is an estimated value whilst the values of longitudinal acceleration and roll rate are measured values. The value of longitudinal acceleration may provide additional information in respect of surface pitch and provide a further source of vehicle acceleration and speed data. The value of roll rate may be helpful in defining a more precise vehicle state model being a model describing motion of the vehicle at a given moment in time.
In some arrangements an intended state of the vehicle (i.e. the state intended by the driver) is determined based upon the steering wheel angle, rate of change of steering wheel angle and road wheel speed. The actual state of the vehicle is determined by reference to a signal from a yaw sensor indicating a rate of yaw of the vehicle. The ESC system calculates a value of positive or negative torque that should be applied to each wheel and then applies the torque to the wheels in order to reduce the difference between the actual state of the vehicle and the state intended by the driver.
ESC systems are generally considered to improve vehicle safety, in particular for novice and inexperienced drivers, by elimination of sideslip. A typical ESC system will for example provide a control input as soon as a deviation from the intended path of travel is detected. Such systems have the potential disadvantage that a driver may never experience sideslip, and thus learn how to manually provide a corrective input.
In some circumstances drivers of motor vehicles on racetracks or skidpans perform controlled sideslip manoeuvres in which they induce sideslip of the vehicle during cornering. If the vehicle is fitted with an ESC system or the like such manoeuvres can be difficult or impossible to perform unless the ESC system is disabled. Disabling of the ESC system is inadvisable since if a driver subsequently finds himself in a situation where he loses control, the ESC system is unable to assist.
WO-A-2009/146826 discloses a modified ESC system whereby a non-zero sideslip angle is allowed for side to side variation of tyre/road friction; a maximum range of 0.5°-8° is mentioned. The purpose of this open loop system is not clear, but it may reduce demand on the ESC control processor. The system is permissive, and delays operation of ESC until a threshold deviation is reached.
It is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known ESC systems, whilst providing for an enhanced driving experience.